Cation Size Effects in Oxygen ion Dynamics of Highly Disordered Pyrochlore-type Ionic Conductors
M.R. Díaz-Guillén; K.J. Moreno; J.A. Díaz-Guillén; A.F. Fuentes; K.L. Ngai; J. García-Barriocanal; J. Santamaría; C. León. Cation Size Effects in Oxygen ion Dynamics of Highly Disordered Pyrochlore-type Ionic Conductors. Physical Review B (ISSN: 1098-0121). 2008, Vol. 78, p. 104304-1-2008.
In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, Edc, in highly disordered pyrochlore-type ionic conductors A2B2O7. Twenty six compositions with the general formulae, Ln2Zr2-yTiyO7, Ln1.7Mg0.3Zr2O7 (Ln = Y, Dy and Gd) and Gd2-yLayZr2O7, were prepared by mechanical milling and their electrical properties measured by using impedance spectroscopy, as a function of frequency and temperature. We also evaluate by using the Coupling Model the effect of cation radii RA and RB, on the microscopic potential-energy barrier, Ea, that oxygen ions encounter when hopping into neighboring vacant sites. We find that for a fixed B-site-cation radius RB, both activation energies decrease with increasing A-site-cation size, RA, as a consequence of the increase in the unit cell volume. In contrast, for a given RA size, the Edc of the Ln2Zr2-yTiyO7 series increases when the average RB size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with RB
In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, Edc, in highly disordered pyrochlore-type ionic conductors A2B2O7. Twenty six compositions with the general formulae, Ln2Zr2-yTiyO7, Ln1.7Mg0.3Zr2O7 (Ln = Y, Dy and Gd) and Gd2-yLayZr2O7, were prepared by mechanical milling and their electrical properties measured by using impedance spectroscopy, as a function of frequency and temperature. We also evaluate by using the Coupling Model the effect of cation radii RA and RB, on the microscopic potential-energy barrier, Ea, that oxygen ions encounter when hopping into neighboring vacant sites. We find that for a fixed B-site-cation radius RB, both activation energies decrease with increasing A-site-cation size, RA, as a consequence of the increase in the unit cell volume. In contrast, for a given RA size, the Edc of the Ln2Zr2-yTiyO7 series increases when the average RB size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with RB